1. Field of Invention
The present invention relates to an air conditioner and a method for controlling an air conditioner.
2. Description of Related Art
Conventional air conditioners have a room temperature sensor, such as a thermistor, and a temperature setting device, such as a remote controller, for setting a room temperature. These air conditioners operate based on the difference between the set room temperature and the room air temperature detected by the room temperature sensor. In an air conditioner having a constant speed compressor, when the detected room air temperature is lower than the set room temperature in a cooling mode, the compressor is turned off. Conversely, when the detected room air temperature is higher than the set room temperature, the compressor is turned on. However, in this type of air conditioner, air conditioning performance is greatly changed when the compressor is turned on and off. Turning the compressor on and off causes the room temperature to change excessively, resulting in uncomfortable air conditioning.
Therefore, an air conditioner having a variable speed compressor has been developed to stabilize the room air temperature at the set room temperature. In this type of air conditioner, the speed of the compressor is controlled by an inverter. A variable frequency output of the inverter is supplied to a compressor motor which drives the compressor. The compressor speed, or output frequency, has been controlled by PI (proportional and integral) control, PID (proportional, integral and differential) control, fuzzy control, GA (genetic algorithms) control, or the like. These controls require two inputs, the difference between the set room temperature and the detected room air temperature, and a room temperature variation at a predetermined period.
In this type of air conditioner, generally, at the time the air conditioner starts, the compressor speed has been determined based only on the difference between the detected room temperature and the set room temperature. After that, the compressor speed has been determined and changed based on the difference between the detected room temperature and the set room temperature, and a room temperature variation between times that the output of the room temperature sensor is determined. The room temperature variation is calculated based on the difference in detected room temperatures before and after a set time between readings of the room temperature sensor.
In this case, the time interval for calculating the room temperature variation was preset as a constant. The time interval was not related to variations of operating mode, such as cooling, heating and dehumidifying, or variations in the direction that air is blown, which is determined by the direction of a louver provided at an air outlet (hereafter called "louver direction") and a volume of conditioned air. However, the temperature of air blown from the air conditioner varies with operating mode, differences in the volume of conditioned air which is controlled by an indoor fan, differences in the louver direction, and so on. Thus, the room temperature variations were detected at time intervals which were uniformly set with no relation to these differences, resulting in poor air conditioner control.
When air conditioning performance is changed, a delay occurs between a change in the actual room temperature and a change in the room temperature detected by the room temperature sensor. When in the heating mode, as shown by FIG. 7, the temperature of the air blown from an indoor unit 10 of the air conditioner is high. Therefore, the air flow in the room may take the path of arrow A shown in FIG. 7. Thus, the effect of a variation in air conditioning performance is quickly detected by room temperature sensor 14 of indoor unit 10 which is mounted in the upper part of the room. When in the cooling mode, as shown by FIG. 8, the temperature of the air from indoor unit 10 is low. Therefore, the cool air sinks, following the path of arrow B, and warm air in the room is taken in by the air conditioner since the warm air rises, following the path of arrow C. Thus, detection of a change in air conditioning performance by room temperature sensor 14 of indoor unit 10 is delayed. Consequently, the relationship between detected room temperature Ta of room temperature sensor 14 and air conditioning performance and time are as shown in FIGS. 9(a) and 9(b). At instant t0, the air conditioning performance is varied in step form, such as when air conditioning starts. During the heating mode, detected room temperature Ta varies upward at instant t2 when time L1 has elapsed from instant t0. However, during the cooling mode, a change in temperature is delayed. Detected temperature Ta does not start to fall until instant t3, a time L2 (&gt;L1) after instant t0.
If room temperature sensor 14 of the air conditioner is positioned in the lower part of the room, the situation is reversed from the case where it is positioned in the upper part of the room. Therefore, the temperature variation during the cooling mode is detected faster than during the heating mode.
As above described, the room temperature sensor's response depends on the air conditioning mode. However, in a conventional air conditioner, the timing at which the room temperature sensor is monitored is a constant interval independent of the air conditioning mode. Therefore, proper operation, or proper compressor speed suitable for the condition of the room, could not be obtained in at least one mode.
The same problem occurs when the louver direction or the air volume is altered. As shown in FIGS. 10(a) and 10(b), when the louver is directed upward during a heating operation, detected room temperature Ta rises at instant t2, which is a small delay from the instant t1 at which the air conditioning performance, or the compressor speed, is varied. When the louver is directed downward, detected room temperature Ta rises at instant t3, which is a larger delay from instant t1.
Also, when the volume of air blown from the air conditioner is "Strong", "Weak" or "Slight", as shown in FIGS. 11(a) and 11(b), detected room temperature Ta rises in the order "Strong", "Weak" and "Slight" at instants t2, t3 and t4 with sequentially larger delays from instant t1 at which the air conditioning performance is varied. Furthermore, the amount of delay between a variation in air conditioning performance and detection by the room temperature sensor differs based on the volume and height of the room to be air conditioned, and differences in the ratings of the air conditioners themselves.
If the time interval at which the room temperature is monitored is set relatively shorter than the delay at which a performance variation is detected by the room temperature sensor, air conditioning operation, e.g., the compressor speed, is further changed before the effect of the initial performance variation is detected by the room temperature sensor. Therefore, the compressor speed is further increased when the previous monitoring cycle caused the compressor speed to increase, and the compressor speed is further decreased when the previous monitoring cycle caused the compressor speed to decrease. Thus, the actual room temperature will vary from the set temperature by a large amount, and a hunting phenomenon will occur.
If the time interval at which the room temperature is monitored is set relatively longer than the delay at which an operation variation is detected by the room temperature sensor, the next adjustment in air conditioner operation occurs at a time when the effect of the previous performance adjustment has become sufficiently apparent, e.g., after the detected room temperature has changed in response. Therefore, the room temperature is stably controlled. However, the time for the room temperature to assume the set temperature increases and the variation of the compressor speed at each time becomes large since the variation in room temperature between the set times becomes large. Thus, the time for the room temperature to reach a comfortable temperature, e.g., the set room temperature, becomes long.